1. Field
The present disclosure relates to a communication system to wirelessly transmit a signal modulated under an OFDM (Orthogonal Frequency Division Multiplexing) scheme, a communication device to which the communication system is applied, and a reception device to receive the wirelessly transmitted signal.
2. Description of the Related Art
In recent years, high-speed communications such as LTE (Long Term Evolution), WiMAX (Worldwide Interoperability for Microwave Access), etc. based on a modulation scheme using OFDM have been put to practical use. OFDM has guard intervals and therefore is highly usable in frequency selective multipath, and the implementation of MIMO (multiple-input and multiple-output) is easier in OFDM than in other modulation schemes. That is, since FFT (Fast Fourier Transform) allows signal processing in a frequency domain to be performed easily, the estimation of a complex channel matrix and the signal separation can be facilitated. Further, multiuser MIMO (MU-MIMO) establishing communications between a single base station and many terminals using the same frequency is considered as its applied technology.
Although MIMO is targeted only for communications from the base station side, that is, downlink at present, researches are being conducted to apply MIMO to transmission from a mobile terminal (uplink) in the future. According to MIMO, high-speed communication is realized by transmitting different signals in parallel from plural antennas, which is, however, based on the premise that there are no delays between all signals and exact temporal agreement is achieved therebetween. In a base station, the agreement can be realized by implementing high-precision hardware. However, when performing MIMO transmission from a mobile terminal, it is difficult to attain accurate temporal agreement for each of plural transmission paths in the terminal. Highly precise synchronization techniques cannot be introduced into a mobile terminal because of its limited size and power consumption. Further, in the case of MU-MIMO, it is difficult to perform MIMO transmission due to a relative delay between terminals occurring in transmission.
For the OFDM reception, there is a need to detect the FFT frame with accuracy. The circumstances where the frame detection is erroneously performed will be considered. When FFT is performed earlier than the original start of a frame and the original start point of the frame is inside a guard interval, the orthogonality of subcarriers after the FFT is retained. However, the guard interval length is reduced and it becomes impossible to avoid desired multipath. On the other hand, when the time of starting FFT is later than the original start point of the frame, an intersymbol interference occurs in the signal stream tail part and the orthogonality between subcarriers is not retained.
Accordingly, many ideas are implemented to acquire synchronization in a receiver. FIG. 1 illustrates a reception device 10 receiving MIMO transmission by applying a modulation scheme using OFDM.
The reception device 10 of FIG. 1 has two reception paths of a path #0 and a path #1. The reception path of the path #0 includes a high frequency unit (hereinafter referred to as an “RF unit”) 12a to which an antenna 11a is connected. A signal received with the RF unit 12a is converted into digital data with an analog-to-digital converter 13a. The data converted with the analog-to-digital converter 13a is supplied to a correlation detection unit 15 via a matched filter 14a configured to detect a preamble. The correlation detection unit 15 detects the head position (synchronization point) of an FFT frame through auto-correlation or cross-correlation. A guard interval elimination unit 16a eliminates a guard interval from a received FFT frame based on the head of the FFT frame, which is detected with the correlation detection unit 15.
Data from which the guard interval is eliminated with the guard interval elimination unit 16a is supplied to an FFT unit 17a, data modulated into individual subcarriers with the FFT unit 17a is retrieved, and the retrieved reception data of the path #0 is supplied to a reception data processing unit 18.
As for the reception path of the path #1, the configuration is the same as that of the reception path of the path #0. That is, a signal received with an RF unit 12b to which an antenna 11b is connected is supplied to an analog-to-digital converter 13b, a matched filter 14b, the correlation detection unit 15, a guard interval elimination unit 16b, and an FFT unit 17b in sequence, and reception data of the path #1 is supplied to the reception data processing unit 18.
In Patent Literature 1, detection of the head of an FFT frame through an auto-correlation operation and a cross-correlation operation is disclosed.